Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder can be operated using four cylinders. Cylinder deactivation improves fuel economy by reducing pumping losses.
To smoothly transition between activated and deactivated modes, the internal combustion engine must produce torque with a minimum of disturbances. Otherwise, the transition will not be transparent to the driver. In other words, excess torque will cause engine surge and insufficient torque will cause engine sag, which degrades the driving experience.
For example for an eight-cylinder engine, intake manifold pressure is significantly lower during eight-cylinder operation than during four-cylinder operation. During the transition from eight to four cylinders, there is a noticeable torque reduction or sagging in four-cylinder operation until the intake manifold reaches a proper manifold pressure level.